In the manufacturing of automobile bodies, multi-component composite coating compositions are applied to vehicle substrates using multiple layers of coatings, including electrophoretically applied primers, one or more primer surfacers, and various color coats and/or clear coats. These coatings not only enhance the appearance of the automobile, but also provide protection from corrosion, chipping, ultraviolet light, acid rain, and other environmental conditions which can deteriorate the coating appearance and damage or corrode the underlying car body substrate.
The formulations of these coatings can vary widely and, hence, the drying and curing conditions may differ for each coating layer, depending on the cure chemistry of the ingredients and the nature of any carrier solvents. Waterborne coatings are becoming more commonplace, and drying conditions are different than for conventional solventborne systems. A major challenge that faces all automotive manufacturers is how to dry and cure these coatings rapidly during vehicle production with minimal capital investment and floor space, which is valued at a premium in manufacturing plants.
Various ideas have been proposed to speed drying and curing processes for automobile coatings, such as hot air convection drying. While hot air drying is rapid, a skin can form on the surface of the coating, which impedes the escape of volatiles from the coating composition and causes pops, bubbles, or blisters which ruin the appearance of the dried coating.
Other methods and apparatus for drying and curing a coating applied to an automobile body are disclosed in U.S. Pat. Nos. 4,771,728; 4,907,533; 4,908,231; and 4,943,447 in which the automobile body is heated with radiant heat for a time sufficient to set the coating on Class A surfaces of the body and subsequently the coating is cured with heated air.
U.S. Pat. No. 4,416,068 discloses a method and apparatus for accelerating the drying and curing of refinish coatings for automobiles using infrared radiation. Ventilation air used to protect the infrared radiators from solvent vapors is discharged as a laminar flow over the car body. FIG. 15 is a graph of temperature as a function of time showing the preferred high temperature/short drying time (curve 122) versus conventional infrared drying (curve 113) and convection drying (curve 114). Such rapid, high temperature drying techniques can be undesirable because a skin can form on the surface of the coating that can cause pops, bubbles, or blisters as discussed above.
U.S. Pat. No. 4,336,279 discloses a process and apparatus for drying automobile coatings using direct radiant energy, a majority of which has a wavelength greater than 5 microns. Heated air is circulated under turbulent conditions against the back sides of the walls of the heating chamber to provide the radiant heat. Then, the heated air is circulated as a generally laminar flow along the inner sides of the walls to maintain the temperature of the walls and remove volatiles from the drying chamber. As discussed at column 7, lines 18-22, air movement is maintained at a minimum in the central portion of the inner chamber in which the automobile body is dried.
U.S. Pat. Nos. 6,113,764; 6,200,650; 6,221,441; 6,231,932; and 6,291,027 disclose multi-stage processes for drying and curing electrodeposited coatings, primers, base coats, and topcoats using various combinations of air drying and infrared radiation.
A rapid, multi-stage drying process for automobile coatings is needed which inhibits formation of surface defects and discoloration in the coating, particularly for use with waterborne base coats to be overcoated with a clear topcoat.